1. Field of the Invention
The present invention relates to a method for driving an organic electroluminescent display device, which uses an organic electroluminescent light emitting element (hereinbelow, referred to as organic electroluminescent element).
2. Discussion of Background
An organic electroluminescent element has an organic thin film provided between an anode and a cathode. The organic thin film, which is sandwiched between both electrodes, has unnegligible capacitance formed therein. The organic electroluminescent element has properties similar to semiconductor light emitting diodes. When the anode side of the thin film is provided on a higher voltage side, and when a certain voltage is applied across both electrodes, the organic electroluminescent element emits light. Conversely, when the cathode side of the thin film is provided on a higher voltage side, the organic electroluminescent element does not emits light since almost no current flows. For this reason, the organic electroluminescent element is also called an organic light emitting diode in some cases.
When a constant voltage is applied across the thin film of an organic electroluminescent element, the luminance of the organic electroluminescent element greatly varies, depending on a change in temperature or a change with time. However, the width of variations in the luminance of an organic electroluminescent element is small with respect to the value of currents. In order to obtain required display intensity, it is common to use a constant-current drive wherein a constant-current circuit is provided in a driving circuit to supply a constant current to respective organic electroluminescent elements.
An organic electroluminescent display device, which has an organic electroluminescent element provided in each of pixels of matrix electrodes, is available. FIG. 9(a) and FIG. 9(b) are a schematic perspective view and a schematic cross-sectional view of the organic electroluminescent display device. There are provided a set of anode strips 2 connected to an anode or forming an anode per se, and a set of cathode strips 1 connected to a cathode or forming a cathode anode per se, which extend in a direction perpendicular to the anode strips. An intersection between a cathode strip 1 and an anode strip 2 forms a pixel, and an organic thin film 3 is sandwiched between both electrodes. In this manner, pixels, which are formed by organic electroluminescent elements, are provided in a matrix fashion and in a planar fashion on a glass substrate 4.
A technique for performing display of an organic electroluminescent display device by passive matrix addressing will be explained. In explanation below, one of the set of the cathode strip 1 and the set of the anode strip 2 works as scanning strips, and the other works as data strips. Respective scanning strips are connected to a scanning driver, which is provided with a constant-current circuit. By this arrangement, constant-current drive is performed with respect to the scanning strips. The scanning strips are sequentially scanned so that one of the scanning strips is in a selected state with a selection voltage applied and the remaining scanning strips are in a non-selected state without the selection voltage applied. In general, the scanning strips are sequentially scanned to have a certain drive voltage applied thereto from the scanning strip at one end of the set of the scanning strips to the scanning strip at the other end so that one scanning strip has the selection voltage applied thereto in every selection period and so that all scanning strips are scanned in a certain period.
The data strips are connected to a data driver, which has a constant-current circuit provided at an output stage. Display data, which correspond to the display pattern of selected scanning strips, are supplied to all data strips in synchronization with the scanning of the scanning strips. A current pulse, which is supplied to the data strips from the constant-current circuit, flows in a selected scanning strip through organic electroluminescent elements, which are located at the intersections between the selected scanning strip and the data strips.
The pixel of an organic electroluminescent element emits light only in a period wherein the scanning strip with that pixel connected thereto is selected and there is current supply from the data strip. When the current supply from the data strip stop, the light emission also stops. While a current supply is made to the organic electroluminescent elements sandwiched between the set of the data strips and the set of the scanning strips in this manner, all scanning strips are sequentially scanned in a repetitive fashion. In accordance with a desired display pattern, the emission and the non-emission of light is controlled with respect to the pixels of the entire display screen.
For driving the organic electroluminescent elements, the set of the anode strips 2 and the set of the cathode strips 1 of the organic electroluminescent elements may be set so that one of the sets works as the scanning strips or the data strips. In other words, the anode strips 2 are used as the scanning strips while the cathode strips 1 are used as the data strips. Or, the anode strips 2 are used as the data strips while the cathode strips 1 are used as the scanning strips. Both sets of the electrodes have interchangeability in terms of driving the organic electroluminescent elements. Generally, it is common that the data scanning strips correspond to the anode strips 2 and the scanning strips correspond to the cathode strips 1. Hereinbelow, explanation of the driving and the display of the organic electroluminescent display device will be made about a case wherein the cathode strips 1 works as the scanning strips and the anode strips 2 work as the data strips. In explanation below, the array of pixels that extend parallel with the scanning strips will be also called “row”, while the array of pixels that extend parallel with the data strips will be also called “column”.
First, the scanning strips, which are connected to the cathode for the organic electroluminescent elements, need to satisfy the following electric potential condition. Specifically, the potential of a scanning strip in the selected state need to be lower than the potential of a scanning strip in the non-selected state. For the purpose, driving is performed so that the potential of a scanning strip in the selected state is set at ground (earth) potential so as to provide a scanning strip in the non-selected state with a higher potential than the ground potential.
The data strips on the column side are supplied with a constant current when output data are turn-on data for turning on a pixel. The data strips on the column side are supplied with a constant voltage equal to ground potential when output data are turn-off data for turning off a pixel. In other words, the data strips are configured so as to be switched between a constant-current output and a constant voltage output, depending on whether a pixel is turned on or off. The reason why the data strips are supplied with the constant current output is that the luminance is controlled by the value of a current as stated earlier.
The direction of a current, which flows in an organic electroluminescent element, is set so that the current flows from the data strip as an anode strip 2 to the scanning strip as a cathode strip 1 through the organic thin film 3. For this reason, the potential of the data strips is set so as to be higher than ground potential as the potential of a scanning strip in the selected state.
As shown in the equivalent circuit diagram of FIG. 10, organic electroluminescent elements exhibit not only an electrical property as diodes but also a capacitive characteristic. By supplying the current into a desired pixel from the data driver having the constant-current circuit, light is emitted from the pixel of an organic electroluminescent element, which is in a row with the selection voltage applied thereto. However, the pixels that are in non-selected rows without the selection voltage applied thereto simultaneously need to be capacitively charged.
When the number of the pixels of an organic electroluminescent element, which are connected to one data strip, increases according to an increase in the number of rows of the matrix forming a display screen, the current required for charging the capacitance of all pixels reaches an unnegligible value. As a result, the current that flows in a pixel in a row with the selection voltage applied thereto decreases to provide the luminance with a lower value than the expected value.
In order to solve this problem, two driving methods have been proposed. A first method is reset driving. When driving is switched from one scanning strip to the next one, all scanning strips are set at an equal potential once, and then charging is started at the equal potential for driving (e.g., JP-A-9-232074, paragraph 0024 to paragraph 0032 and FIG. 1 to FIG. 4).
The second method is precharge driving. A charging circuit is additionally provided on the data driver side, and the respective pixels of an organic electroluminescent element are precharged for a certain time period. The luminance is improved by increasing the driving voltage for the organic electroluminescent elements (e.g., JP-A-11-45071, paragraph 0022 to paragraph 0029 and FIG. 2).
Hereinbelow, previously setting all scanning strips at an equal potential once or previously charging the respective pixels of an organic electroluminescent element will be referred to “the capacitive charge”.
FIG. 12 shows a basic driving waveform in a case wherein the display pattern shown in FIG. 11 is displayed on a 4×4 matrix display screen having pixels positioned in columns C1, C2, C3 and C4 and in rows R1, R2, R3 and R4. Now, the driving method wherein the time width of an output current pulse from the data driver is modified will be explained.
As shown in FIG. 12, the current pulse is supplied so as to have a pulse width occupying substantially the full width of the selection period with respect to a pixel, which is required to emit light with the maximum luminance (a luminance of 100%). The current pulse is supplied so as to have a pulse width occupying a half width in comparison with the case of a luminance of 100% with respect to a pixel, which is required to emit light with a luminance of 50%. This driving method is called a pulse width modulation (hereinbelow, also referred to as PWM).
In the structure of an organic thin film 3 wherein a light emitting layer has a hole transport layer provided on the anode side of in layer, and wherein the hole transport layer and the anode have a hole injection layer interposed therebetween in layer, the hole injection layer may be made of copper phthalocyanine. It has been reported that the hole injection layer can be made of an organic polymeric material to improve the property of an organic electroluminescent display (e.g., JP-A-2000-36390).
In the conventional driving methods, pixels are actually driven after capacitive charge. When the voltage that is applied to the pixels at the time of completion of capacitive charge (charged voltage) fails to reach the voltage that is applied to the data strips at the time of driving a pixel (driving voltage), the difference between the charged voltage and the driving voltage causes a decrease in luminance in some cases. FIG. 13(a) shows an example of the applied voltage, which is applied to a pixel to emit light with a luminance of 100% or a luminance of nearly 100%. In FIGS. 13(a) and 13(b), the time period for supplying a constant current is indicated in the horizontal direction, and an applied voltage is indicated in the vertical direction. The rising edge of each applied voltage is the time when capacitive charge has been completed.
When the charged voltage has the same value as the driving voltage as shown in FIG. 13(a), selected pixels have a desired current immediately flowing therethrough. However, when the charged voltage is lower than the driving voltage as shown in FIG. 13(b), other pixels in the same column that are not selected also have a current flowing therethrough even after completion of capacitive charge until the applied voltage has reached the value of the driving voltage. As a result, the pixels to emit light are short of electric charges, lowering the luminance. When the charged voltage is higher than the driving voltage, the other pixels in the same column that are not selected also have a current flowing out thereof into the selected pixels even after completion of capacitive charge. As a result, the selected pixels have an excessive amount of electric charges, increasing the luminance.
Since the cathode strips 1 have a certain level of resistance, the amount of the current that flows into the cathode strips varies depending on the number of pixels to emit light per one row. As a result, the cathode potential varies depending on the kind of a display pattern. Even when pixels emit light with a relatively high luminance, such as a luminance of 100% or a luminance of nearly 100%, chrominance non-uniformity is. caused in a horizontally striped shape according to a display pattern, depending on the kind of a display pattern and the difference between the charged voltage and the driving voltage, as shown in FIG. 14(b). This type of display state is called horizontal cross-talk. FIG. 14(b) shows a case wherein although an attempt is made to turn off a portion of the display screen and emit light from the remaining portions with a luminance of 100% as shown in FIG. 14(a), the luminance becomes darker than expected since the cathode potential in a row having a large number of pixels to turn on increases to prevent a certain current from flowing the organic electroluminescent elements forming the pixels to turn on.
When light emission is made with a low luminance by PWM, the problem of horizontal cross-talk becomes a big issue. FIGS. 15(a) and 15(b) show examples of the applied voltage for turning on a pixel by PWM. In FIGS. 15(a) and 15(b), the time period for supplying a constant current is indicated in the horizontal direction, and each applied voltage is indicated in the vertical direction.
When the charged voltage has the same value as the driving voltage as shown in FIG. 15(a), selected pixels have a desired current immediately flowing therethrough. However, when the charged voltage has a different value from the driving voltage as shown in FIG. 15(b), other pixels in the same column that are not selected also have a current flowing therethrough even after completion of capacitive charge until the applied voltage has reached the value of the driving voltage. When a pixel is energized to emit light with a low luminance as shown in FIG. 15(b), the time period for supplying a current to the relevant data strip ends before the applied voltage has reached the same value as the driving voltage. In this case, the pixel emits light with a lower luminance than a desired luminance (required luminance). When all pixels have the same current-voltage characteristics in an organic electroluminescent display device, the luminance of the device uniformly lowers over the entire screen. However, in a case wherein the pixels have different current-voltage characteristics, the respective pixels have different values of currents flowing therethrough to fail to provide a uniform luminance over the entire screen even when the pixels have the same voltage applied thereacross. The current-voltage characteristics of a pixel means the relationship between a voltage applied to a pixel and a current flowing through the pixel.
In a case wherein there are variations in the current-voltage characteristics, i.e., wherein pixels have different values of currents flowing therethrough by application of a single voltage, a pixel emits light with the required luminance and another pixel emits light with a lower luminance in spite of that all pixels to emit light are energized so as to emit light with the same luminance by constant-current drivel. As a result, there is caused chrominance non-uniformity wherein the luminance varies to portion from portion to such degree that can be visually recognized.
The degree of the horizontal cross-talk generated becomes greater than a case wherein desired pixels are energized to emit light with a luminance of 100% or a relative high luminance near to a luminance of 100%.
When capacitive charge is performed to all pixels in an organic electroluminescent display, additional power is required for capacitive charge. Even when a display pattern needs a small number of pixels to emit light, the power consumption for the organic electroluminescent display cannot be reduced to a lower value than the power consumption required for capacitive charge.